26-DAY MODULATION OF THE SKY BACKGROUND LY-ALPHA BRIGHTNESS - ESTIMATING THE INTERPLANETARY HYDROGEN DENSITY

We present here a series of sky background observations, all having a fixed pointing direction for at least one solar rotation, made with bo th Voyager UVS instruments between 1981 and 1993. These data show modu lations with a 26 day period, which we have used to estimate the inter planetary hydrogen density. A comparison of these data with multiple s cattering computations of the damping of the solar Ly alpha flux modul ation yields an interplanetary hydrogen density of 0.15 +/- 0.10 cm(-3 ). This result is independent of instrument calibration, but its accur acy is limited by the difficulty of estimating the solar Ly alpha line flux in every direction at the time of observations. To estimate the solar H Ly alpha line flux, we have used the Solar Mesosphere Explorer H Ly alpha flux measurements between 1981 and 1988 and the He I 1083 nm equivalent width data after 1988. We find also that a linear relati on between the integrated H Ly alpha line and the line center does not apply near the solar maximum. Voyager 2 data obtained in 1989 are in better agreement with an empirical nonlinear relation derived by Vidal -Madjar. Finally, for the present position of the Voyager spacecraft, we estimate that the factor by which the modulation is damped is at le ast 0.4, which means that the antisolar sky background modulations hav e an amplitude less than 40% of the amplitude of the subspacecraft sol ar flux at line center.